Among the conventionally-known rotary lawn mowers are ones which cut grass by rotating a cutter blade, accommodated in a downwardly-opening housing, along the grass on a lawn. The rotation of the cutter blade would produce unwanted noise sound, and thus, improvement of cutter blades has been under way with a view to suppressing unwanted noise of the lawn mowers. One example of such lawn mowers is disclosed in Japanese Patent Application Laid-Open Publication No. 2006-67918 (hereinafter referred to as “Patent Literature 1”).
The lawn mower disclosed in Patent Literature 1 includes a rotation shaft extending in a vertical (up-down) direction in a housing, and upper and lower cutter blades accommodated within the housing for rotation about the rotation shaft. The upper cutter blade has a blade section formed along the front edge, in the rotating direction, of the upper cutter blade for cutting grass on a lawn. The lower cutter blade has, on each of its longitudinally opposite end portions, a blade section formed along the front edge, in the rotating direction, of the lower cutter blade, and an air lift section extending from the front edge while curving obliquely rearward and upward for producing swirling air flows and ascending air flows. Production of unwanted noise sound can be suppressed by the air lift sections provided on the lower cutter blade.
In recent years, there has been a growing demand for more consideration or attention to environment during grass cutting operation by the lawn mower. Thus, it has been required nowadays to place more emphasis on noise suppression performance of the cutter blade.
Japanese Utility Model Application Laid-Open Publication No. H01-121323 (hereinafter referred to as “Patent Literature 2”) discloses a cutter blade of a lawn mower constructed to achieve an enhanced noise suppression performance. The cutter blade disclosed in Patent Literature 2 has, on each of its longitudinally opposite end portions, an air lift section and a vertical wing. During the grass cutting operation, the vertical wing is revolvable about the rotation shaft as the cutter blade rotates and extends upward from the outer peripheral edge of the cutter blade. Such a vertical wing suppresses production of a vortex current of air to thereby suppress production of noise sound.
However, because each of the vertical wings extends upward from the outer peripheral edge of the cutter blade, grass cut during the grass cutting operation (grass clippings) may pile up on the upper surface of the cutter blade. Further, each of the vertical wings may be directly exposed to an air flow field, i.e. swirling air flows. Therefore, more improvement has to be made in order to further enhance the noise suppression performance of the lawn mower.